WILEY    ENGINEERING    SERIES  — 3 

MICROSCOPIC 
EXAMINATION  OF   STEEL 


BY 


HENRY    FAY,  Ph.  D.,  D.  Sc. 

Professor  of  Analytical  Chemistry,  Massachusetts  Institute  of  Technology 
Consulting  Metallurgist,  Water  town  Arsenal 


FIRST  EDITION 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LONDON:   CHAPMAN   &   HALL,   LIMITED 
1917 


COPYRIGHT,  1917, 

BY 
HENRY  FAY  A. 


Stanhope  press 

P.    H.GILSON   COMPANY 
BOSTON,  U.S.A. 


T7  F  3 


NOTE. 

This  book  is  being  published  by  permission  of  the  Chief  of 
Ordnance,  United  States  Army. 


PREFACE. 

The  material  contained  in  this  small  volume  was  originally 
issued  by  the  Ordnance  Department  U.  S.  A.  and  represented 
the  results  of  investigative  work  carried  on  at  the  Watertown 
Arsenal.  It  was  intended  for  the  exclusive  use  of  inspectors  of 
ordnance  material.  The  demand  for  the  pamphlet  however 
was  considerable,  and  it  was  thought  that  it  might  serve  a 
wider  field  of  usefulness  by  being  offered  to  others  interested 
in  the  inspection  of  steel.  It  is  in  no  sense  to  be  considered  a 
text  book,  and  it  is  meant  to  present  a  mere  outline  of  metal- 
lographic  methods  illustrating  typical  examples,  and  should  be 
used  only  in  conjunction  with  a  study  of  text  books  on  the 
metallography  of  iron  and  steel  and  with  reference  to  original 
papers  appearing  in  the  journals  bearing  on  this  subject.  It 
should,  furthermore,  be  used  with  caution,  as  it  is  believed  that 
metallographic  diagnosis  can  be  made  only  after  much  ex- 
perience in  the  handling  of  a  variety  of  material.  This  volume 
is  intended  particularly  for  those  who  are  in  need  of  some  help 
in  the  interpretation  of  results. 

The  author  wishes  to  express  his  appreciation  of  "the  kindly 
interest  which  Colonel  Wheeler,  the  Commandant  of  the  Water- 
town  Arsenal,  has  shown  in  this  as  well  as  in  other  work  of 
similar  nature. 

HENRY  FAY. 

CAMBRIDGE,  MASS. 
November,  1916. 


3C4G91 


WILEY   ENGINEERING  SERIES. 


The  Wiley  Engineering  Series  will  embrace  books  devoted 
to  single  subjects.  The  object  of  the  series  is  to  place  in  the 
hands  of  the  practicing  engineer  all  the  essential  information 
regarding  the  particular  subject  in  which  he  may  be  interested. 
Extraneous  topics  are  excluded,  and  the  contents  of  each  book 
are  confined  to  the  field  indicated  by  its  title. 

It  has  been  considered  advisable  to  make  these  books  manuals 
of  practice,  rather  than  theoretical  discussions  of  the  subjects 
treated.  The  theory  is  fully  discussed  in  text  books,  hence  the 
engineer  who  has  previously  mastered  it  there,  is,  as  a  rule,  more 
interested  in  the  practice.  The  Wiley  Engineering  Series  there- 
fore will  present  the  most  approved  practice,  with  only  such 
theoretical  discussion  as  may  be  necessary  to  elucidate  such 
practice. 


Microscopic  Examination  of  Steel 


INTRODUCTION. 

The  supreme  test  of  any  metal  is  a  test  to  destruction,  but 
unfortunately  this  method  is  usually  too  expensive  and  time 
consuming  to  lend  itself  to  useful  purposes.  A  practical  field 
test  is  also  satisfactory,  but  in  order  to  carry  on  such  a  test  in  a 
systematic  manner  a  certain  amount  of  information  must  be  ob- 
tained previous  to  the  assembling  of  the  materials  for  such  a 
test.  In  order  to  judge  whether  or  not  a  metal  is  suitable  for 
the  purpose  for  which  it  is  intended  separate  parts  of  the  metal 
are  selected  for  test  and  upon  the  results  of  these  tests  the  metal 
is  judged  as  a  whole.  Physical  and  chemical  tests  are  usually 
applied  and  both  give  satisfactory  and  necessary  information 
within  certain  limits  but  neither  is  absolutely  conclusive  in 
itself  and  the  results  of  the  two  methods  of  testing  should  al- 
ways be  used  in  conjunction. 

To  further  supplement  the  information  obtained  by  physical 
and  chemical  tests  metallographic  tests  which  give  information 
not  obtainable  by  the  other  methods  have  been  introduced.  The 
particular  use  of  metallographic  testing  is  to  give  information 
in  regard  to  the  homogeneity  of  the  metal,  and  the  heat  treat- 
ment to  which  it  has  been  subjected.  These  observations  are 
extremely  useful  in  conjunction  with  the  results  of  the  other 
methods,  but  are  not  sufficient  in  themselves  at  the  present  time 
as  a  basis  for  conclusion,  on  account  of  not  yet  having  been 
sufficiently  standardized.  Interpretation  of  results  is  an  im- 
portant factor,  and  in  cases  of  doubt  the  results  should  be  re- 
ferred to  someone  having  had  experience. 

The  following  pages  are  submitted  as  a  help  to  inspectors, 
who  should  inform  themselves  more  fully  in  regard  to  underlying 
principles  involved  and  to  methods  employed. 


2  MICROSCOPIC  EXAMINATION  OF  STEEL 

SLOWLY  COOLED   STEELS. 

When  steel  passes  from  the  liquid  to  the  solid  state  it  forms 
what  is  called  a  solid  solution.  This  solid  solution  is  usually 
referred  to  as  Austenite.  The  temperature  at  which  the  change 
from  liquid  to  solid  metal  takes  place  is  progressively  lowered 
with  increase  of  carbon.  The  resulting  solid  solution  is  sub- 
ject to  certain  reactions  which  take  place  as  the  solid  mass  is 
further  cooled.  The  temperatures  at  which  these  reactions  :ake 
place  are  known  as  critical  temperatures,  or  critical  points,  and 
are  made  evident  by  evolutions  or  absorptions  of  heat.  The 
reactions  are  reversible  ones,  depending  upon  whether  the  points 
are  observed  on  a  rising  or  falling  temperature.  Points  ob- 
tained upon  a  falling  temperature  are  referred  to  a^  Ar3,  Ar2, 
Ari;  points  obtained  on  a  rising  temperature  as  Aci,  Ac2,  and 
Ac3. 

The  reactions  which  take  place  at  these  critical  temperatures 
determine  the  character  of  the  resulting  product.  Inasmuch  as 
some  of  these  points  are  dependent  upon  the  percentage  compo- 
sition of  the  metal,  particularly  carbon,  the  properties  of  any 
steel  are  then  dependent  upon  the  percentage  of  carbon  and  the 
completeness  with  which  the  reactions  have  taken  place  at  the 
critical  temperatures. 

For  practical  purposes,  the  Ar2  and  Ac2  points  need  not  be  con- 
sidered. In  steels  containing  less  than  0.85  per  cent  carbon,  the 
Ar3  point  represents  the  beginning  of  the  decomposition  of  solid 
solution  and  the  throwing  out  of  some  of  the  solvent,  viz.,  iron. 
The  temperature  at  which  this  reaction  begins  is  progressively 
lowered  with  increase  of  carbon  up  to  0.85  per  cent.  In  steels 
•-  ,-fc  containing  more  thar^o.^per  cent  carbon  the  upper  critical  tem- 
perature is  progressively  raised'  with  increase  of  carbon  up  to 
1.7  peV  cent  carbon  and  is  referred  to  as  Acm,  and  the  reaction 
taking  place  represents  the  beginning  of  the  separation  of  iron 
carbide  (Fe3C)  on  a  falling  temperature,  and  the  completion  of 
solution  of  iron  carbide  on  a  rising  temperature.  The  Ari  point 
for  all  carbon  steels  is  approximately  at  the  same  temperature  and 
represents  the  final  decomposition  of  the  solid  solution  into  its 


MICROSCOPIC  EXAMINATION  OF  STEEL  3 

constituents,  iron  and  iron  carbide.  The  Aci  point  is  approxi- 
mately 30°  higher  than  the  Ari  point,  and  represents  the  begin- 
ning of  the  formation  of  solid  solution.  In  steels  containing 
0.85  per  cent  carbon  there  is  but  one  critical  temperature  at  which 
the  solid  solution  decomposes  into  its  constituents,  iron  and 
iron  carbide.  This  is  known  as  an  eutectoid  steel,  and  its  mi- 
croscopic constituent  is  pearlite.  Pearlite  always  etches  dark, 
and  under  high  magnifications  is  shown  to  consist  of  alternate 
lamjnations  of  iron  and  iron  carbide.  Steels  containing  less 
than  0.85  per  cent  carbon  are  hypoeutectoid,  and  contain  the 
microscopic  constituents  ferrite  and  pearlite,  the  amount  of  the 
latter  increasing  with  the  per  cent  of  carbon.  Steels  containing 
more  than  0.85  per  cent  carbon  are  hypereutectoid,  and  contain 
the  microscopic  constituents  cementite  (iron  carbide,  FesC)  and 
pearlite.  These  facts  are  illustrated  in  the  diagram.  (Fig.  i.) 

Hypoeutectoid  steels  are  used  for  forgings  and  castings;  hy- 
pereutectoid steels  for  tools;  eutectoid  steels  for  tires,  springs, 
etc.  Examples  illustrating  these  constituents  are  shown  in 
photographs  Nos.  1-9. 

Photograph  No.  i  shows  the  characteristics  of  a  carbonless 
iron  in  which  the  whole  field  is  made  up  of  granules  of  ferrite. 
With  the  introduction  of  carbon  there  is  a  decrease  in  the  amount 
of  ferrite  and  the  constituent  pearlite  begins  to  appear.  This 
is  shown  in  photograph  No.  2  for  a  steel  containing  0.18  per  cent 
carbon.  Inasmuch  as  steels  containing  0.85  per  cent  carbon  con- 
tain ICQ  per  cent  pearlite,  and  as  pearlite  is  composed  of  ferrite 
and  cementite,  in  this  steel  containing  0.18  per  cent  carbon  there 
ought  to  be  approximately  20  per  cent  of  the  field  made  up  of 
pearlite.  In  a  steel  containing  0.5  per  cent  carbon  there  should  be 
58.8  per  cent  pearlite.  Such  a  steel  is  shown  in  photograph 
No.  4.  A  eutectoid  steel  containing  0.83  per  cent  carbon  and 
practically  100  per  cent  pearlite  is  shown  in  photograph  No.  7. 
Under  higher  magnifications  pearlite  shows  the  laminated  struc- 
ture shown  in  photograph  No.  8.  In  hypereutectoid  steels  the 
excess  carbon  beyond  that  necessary  to  form  pearlite,  separates 
as  free  cementite  and  the  amount  of  free  cementite  can  be  cal- 
culated. Inasmuch  as  the  whole  field  is  made  up  of  pearlite 


4  MICROSCOPIC  EXAMINATION  OF   STEEL 

and  cementite  and  it  requires  0.85  per  cent  of  the  carbon  to 
form  the  pearlite,  the  difference  between  the  total  carbon  and 
0.85  per  cent  carbon  gives  the  amount  of  carbon  available  to  form 
cementite.  Thus  in  a  steel  containing  1.25  per  cent  carbon  there 
is  available  for  cementite  formation  0.4  per  cent  carbon  and  as 
cementite,  Fe3C,  contains  6.67  per  cent  carbon  the  0.4  per  cent 
carbon  would  furnish  approximately  6  per  cent  cementite  which 
would  occur  in  the  free  state,  and  the  steel  would  contain  94  per 
cent  of  pearlite  and  6  per  cent  cementite.  See  photograph  No.  9. 
It  is  thus  evident  that  by  microscopic  examination  a  rough 
determination  of  carbon  may  be  made  on  slowly  cooled  steels. 
Photograph  No.  10  shows  a  steel  containing  1.46  per  cent  car- 
bon —  approximately  9  per  cent  cementite  —  in  which  the  white 
constituent  is  cementite. 

RAPIDLY  COOLED   STEELS. 

Every  chemical  reaction  has  a  time  factor.  In  the  slow  cooling 
or  heating  of  steels  the  reactions  taking  place  at  the  critical 
temperatures  are  almost  complete  and  the  products  of  the  re- 
actions are  in  a  state  of  stable  equilibrium.  If  this  time  factor 
be  disturbed,  the  state  of  equilibrium  is  also  disturbed.  If  the 
rate  of  cooling  is  rapid  enough,  the  reactions  which  ordinarily 
take  place  on  slow  cooling  are  suppressed  and  the  metal  is  main- 
tained in  the  condition  in  which  it  existed  at  the  temperatures 
from  which  cooling  began.  If  cooled  rapidly  from  above  Ari, 
the  reactions  which  ordinarily  take  place  on  slow  cooling  have  not 
time  to  take  place  and  the  solid  solution  present  persists  at  the 
ordinary  temperatures,  but  inasmuch  as  solid  solution  of  iron 
carbide  in  iron  is  in  stable  equilibrium  only  at  temperatures  above 
Aci,  then  it  must  be  in  a  state  of  unstable  equilibrium  at  the 
ordinary  temperature.  This  is  the  condition  of  hardened  steel 
and  is  produced  by  quenching  in  water,  brine,  or  oil  at  some 
temperature  above  Aci.  The  tendency  toward  stable  equilibrium 
is  very  small  at  the  ordinary  temperature,  but  it  increases  with 
rise  of  temperature  and  time.  The  microscopic  constituent  pro- 
duced in  quenched  steel  is  known  as  martensite  and  is  character- 
ized by  needlelike  crystals  crossing  each  other  at  angles  of  60°. 


MICROSCOPIC  EXAMINATION  OF  STEEL  5 

Photograph  No.  n  shows  the  characteristic  appearance.  Com- 
mercially martensitic  steels  are  unimportant  on  account  of  their 
extreme  brittleness  and  they  are  found  only  rarely. 

More  important,  however,  are  those  steels  in  which  the  un- 
stable state  of  equilibrium  has  been  partially  relieved  by  the 
process  known  as  tempering  or  drawing. 

If  heated  to  temperatures  below  Aci,  the  effect  produced  will 
vary  with  the  temperature.  The  lower  the  temperature  of  re- 
heating the  smaller  is  the  tendency  toward  stable  equilibrium; 
the  higher  the  temperature  the  more  stable  is  the  product  pro- 
duced. Practically  any  degree  of  hardness  may  be  produced, 
varying  from  glass-hard  to  extreme  softness,  by  properly  regu- 
lating the  temperature  of  the  heating.-  There  are  then  two 
extremes,  the  martensitic  steels  formed  by  quenching  from  above 
Aci  and  the  pearlitic  steels  formed  by  cooling  slowly  from  Ari  or 
above.  Intermediate  between  these  extremes  are  steels  in  which 
more  or  less  of  the  unstable  solid  solution  has  been  partly  con- 
verted to  more  stable  forms.  Such  forms  are  represented  mi- 
croscopically by  troostite  and  sorbite.  Troostite,  formed  by 
tempering  at  relatively  low  temperatures,  represents  the  first 
stage  in  the  breaking  down  of  the  unstable  equilibrium  and  is 
characterized  by  the  amorphous  or  slightly  granular  structure. 
On  etching  with  alcoholic  nitric  acid  the  surface  usually  blackens 
immediately  from  separated  carbon.  Photograph  No.  12  shows 
troostite. 

Sorbitic  steels  are  those  which  have  been  quenched  and  heated 
to  a  higher  temperature  and  thus  represent  a  more  stable  state 
of  equilibrium.  They  are  characterized  by  having  a  peculiar 
grayish  color,  and  the  structure  is  better  defined  than  troostite. 
The  particles  of  iron  carbide  have  not  assumed  definite  lami- 
nations with  ferrite  as  in  pearlite,  but  are  very  much  more  dis- 
tinct than  in  troostite.  Photograph  No.  13  shows  sorbite. 

Similar  effects  may  be  produced  by  varying  the  rate  of  cooling. 
Thus,  if  the  steel  be  quenched  in  the  critical  range  at  the  be- 
ginning of  the  transformation  of  solid  solution  into  its  compo- 
nents, troostite  may  be  produced;  if  quenched  near  the  end  of 
the  transformation,  sorbite  may  be  produced.  Quenching  be- 


6  MICROSCOPIC  EXAMINATION  OF  STEEL 

low  the    critical  range  should  theoretically  produce  the  same 
effects  as  slow  cooling. 

ANNEALED   STEELS. 

In  the  heating  of  steels  to  a  high  temperature,  and  in  the  slow 
cooling  from  a  high  temperature  either  without  having  been 
forged  or  after  having  been  forged  at  a  high  temperature,  there 
is  considerable  grain  growth  of  the  steel  with  strong  tendency 
toward  granulation.  In  the  making  of  castings  there  is  good 
opportunity  for  grain  growth  and  also  for  the  formation  of  inter- 
nal strains.  To  remove  strains  and  .to  refine  the  grain,  steel  is 
annealed.  The  temperature  of  annealing  is  determined  by  the 
percentage  of  carbon.  For  hypoeutectoid  steels  the  tempera- 
ture most  suitable  is  coincident  with  the  temperature  at  which 
complete  diffusion  or  solution  of  the  constituents  takes  place. 
This  corresponds  to  the  Ac3  point,  and  for  steels  containing  less 
than  0.85  per  cent  carbon  is  progressively  lowered  from  900°  C.  for 
pure  iron  to  700°  C.  for  eutectoid  steel.  Usually  a  temperature 
25°  to  50°  higher  should  be  reached. 

In  hypereutectoid  steels  the  tendency  for  the  structure  to 
coarsen  is  very  much  greater,  and  consequently  the  heating  for 
annealing  should  be  carried  only  a  little  over  the  Aci  point. 

Microscopic  examination  gives  a  clew  not  only  to  the  tempera- 
ture at  which  a  steel  has  been  annealed,  but  also  gives  a  very 
strong  indication  of  the  rate  at  which  cooling  has  taken  place. 

NON-METALLIC   IMPURITIES. 

In  addition  to  the  normal  constituents  of  steel  there  are  some 
abnormal  constituents  commonly  classified  under  the  name  of 
slag,  which  may  include  such  substances  as  silicate  of  manganese, 
sulphide  of  manganese,  and  oxide  of  iron.  Mixtures  of  sili- 
cates of  iron  and  manganese  may  occur  and  also  mixtures  of 
silicate  of  manganese  and  sulphide  of  manganese.  Oxide  of  iron 
ordinarily  occurs  near  the  surface  as  the  result  of  rolling  in  some 
mill  scale,  but  may  at  times  penetrate  quite  deeply  into  the  in- 
terior, and  in  either  case  is  usually  accompanied  by  partial 
decarbonization. 


MICROSCOPIC  EXAMINATION  OF  STEEL  7 

It  is  difficult  at  times  to  distinguish  readily  between  silicate 
and  sulphide  of  manganese  without  resort  to  special  tests.  They 
both  occur  as  dove-colored  constituents  and  are  best  observed 
on  the  polished  but  unetched  surface.  See  photographs  Nos. 
17  and  18.  The  former,  especially  if  occurring  in  masses,  will 
frequently  show  crystalline  etch  figures.  See  photographs  Nos. 
14  and  15.  In  slowly  cooled,  unworked  steel  they  will  show  as 
rounded  dove-colored  masses  usually  in  the  ferrite;  in  forged 
steels  they  are  elongated  in  the  direction  of  forging.  See  photo- 
graphs Nos.  1 6  and  17.  Inasmuch  as  all  steels  contain  some 
slag,  it  is  a  matter  of  opinion  as  to  how  much  should  be  allowed. 
The  form  in  which  it  occurs  is  of  greater  importance  than  the 
actual  amount  present,  although  the  smaller  the  amount  of  slag 
the  better  the  product.  It  is  especially  undesirable  to  have  the 
slag  in  masses,  and  particularly  in  elongated  streaks.  It  then 
produces  ghost  lines  and  is  usually  accompanied  by  streaks  of 
ferrite  which  are  usually  further  embrittled  by  the  presence  of 
high  phosphorus.  Such  slag  streaks  are  frequently  the  source 
of  cracks  which  develop  along  the  slag  in  the  direction  of  forging. 

By  reference  to  photographs  Nos.  14  to  18  may  be  seen  some 
of  the  forms  in  which  slag  may  occur,  and  other  examples  may 
be  seen  under  the  discussion  of  special  cases. 

Inspectors  should  observe  the  following: 

(1)  Macrostructure  for  (a)  ingot  structure,  (b)  segregation, 
and  (c)  blowholes. 

(2)  Slag. 

(3)  Streaks. 

(4)  Heat  treatment. 

(5)  Composition. 

MACROSTRUCTURE. 

Macrostructure  is  the  appearance  of  the  etched  surface  seen 
by  the  naked  eye.  Micros  true  ture,  on  the  other  hand,  is  that 
appearance  of  the  etched  surface  seen  under  a  microscope.  The 
macrostructure  often  yields  important  evidence  concerning  the 
qualities  of  the  metal.  The  structure  is  usually  developed  on 


8  MICROSCOPIC  EXAMINATION  OF  STEEL 

a  roughly  polished  area  of  considerable  size,  or  if  it  is  possible 
to  work  only  with  small  sections,  it  is  recommended  that  the 
test  bars  be  rough-polished  on  two  sides,  at  right  angles  to  each 
other,  before  being  turned  into  shape.  The  development  of  the 
structure  is  accomplished  by  etching  the  surface  with  a  6  per 
cent  solution  of  iodine  in  alcohol  or  with  an  8  per  cent  solution 
of  copper  ammonium  chloride.  The  former  should  always  be 
prepared  immediately  before  use,  as  it  keeps  in  good  condition 
only  a  relatively  short  time.  The  copper  ammonium  chloride 
should  be  used  only  for  those  specimens  which  can  be  dipped 
into  it,  and  the  time  of  action  should  be  approximately  i  minute. 
This  solution  will  not  give  results  with  alloy  steels  on  account  of 
the  tenacity  with  which  the  deposited  copper  adheres  to  the  sur- 
face. With  carbon  steels  the  deposited  copper  is  very  easily 
removed  by  rubbing  with  a  piece  of  wet  cotton. 

In  using  the  iodine  the  polished  surface  is  swabbed  with  cotton 
holding  the  reagent,  and  the  process  is  repeated,  as  fast  as  the 
color  of  the  iodine  disappears,  for  a  period  of  five  minutes. 
The  macrostructure  thus  developed  shows  ingot  structure,  seg- 
regation of  carbon  or  phosphorus,  excessive  slag,  strained  metal. 

The  presence  of  ingot  structure  such  as  shown  in  photograph 
No.  19  indicates  that  the  heat  treatment  and  forging  have  not 
been  sufficient  to  wipe  out  the  crystallization  which  occurred  on 
the  solidification  of  the  metal.  It  is  not  definitely  known  that 
the  presence  of  ingot  structure  is  harmful,  and  it  is  frequently 
accompanied  by  excellent  microstructure,  but  it  serves  as  an 
indication  of  the  heat  treatment  to  which  the  metal  has  been  sub- 
jected after  solidification.  It  is  believed  that  such  a  structure 
would  not  be  found  in  the  best  steel,  and  it  is  further  believed 
that  metal  showing  ingot  structure  would  be  more  liable  to 
rupture  from  shock  than  metal  in  which  the  ingot  structure 
had  been  wiped  out  by  heat  treatment  and  forging. 

Segregation  of  carbon  or  phosphorus  may  be  brought  out  most 
successfully  by  etching  a  complete  cross  section  of  the  metal 
under  investigation,  such  as  shown  in  photograph  No.  20  of  a 
rail  section  in  which  segregation  of  carbon  is  shown,  or  in  photo- 
graph No.  21  of  a  cross  section  of  a  piece  of  cold-rolled  shafting 


MICROSCOPIC   EXAMINATION  OF   STEEL  9 

in  which  segregation  of  phosphorus  is  shown.  Photograph  No. 
22  shows  a  longitudinal  section  of  the  same  piece  of  shafting  and 
is  etched  with  nitric  acid,  which  darkens  the  phosphide  areas. 
It  is  seldom  possible,  however,  to  obtain  complete  cross  sections, 
but  valuable  indications  may  be  obtained  from  surface  struc- 
tures or  from  test-bar  sections,  such  as  shown  in  photograph 
No.  38  of  test  bars  marked  2,  3,  and  5.  The  phenomena  ob- 
served here  will  be  more  fully  discussed  later. 

Both  iodine  and  copper  ammonium  chloride  show  dark  areas 
where  segregated  carbon  occurs;  iodine  leaves  phosphide  areas 
whiter  than  the  surrounding  metal,  while  copper  ammonium 
chloride  leaves  the  area  dark,  and  the  copper  adheres  more  ten- 
aciously to  a  phosphide  than  to  a  carbide  area.  Excessive 
slag  is  shown  by  a  pitted  or  spongy  area  where  the  slag  has  seg- 
regated. 

Strained  metal  invariably  etches  more  darkly  than  metal 
which  has  not  been  strained,  and  the  extent  of  the  strained  area 
may  be  determined  by  etching  with  either  reagent.  Photo- 
graph No.  23  shows  an  area  which  has  been  strained  by  cold 
work. 

SLAG. 

Slag  is  undoubtedly  harmful  if  the  material,  either  silicate  or 
sulphide,  occurs  in  large  quantities  in  segregated  masses,  or  if 
present  in  masses  elongated  in  the  direction  of  forging.  The 
position  of  the  slag  in  the  metal  is  also  of  importance,  and  that 
which  occurs  in  regions  of  severest  strain  is  most  to  be  feared. 
It  must  always  be  remembered  that  in  observing  slag  the  par- 
ticular spot  in  view  is  magnified  50  to  100  diameters,  and  it  may 
be  only  an  isolated  spot.  If  found,  an  exploration  in  the  neigh- 
boring region  should  be  made  to  see  if  it  is  generally  distributed. 
It  should  also  be  remembered  that  low-carbon  steels  may  carry 
with  safety  more  slag  than  high-carbon  steels.  In  photographs 
Nos.  14,  15,  16,  17,  53,  54,  and  55  are  shown  slag  areas  which 
would  at  once  arouse  suspicion,  and  if  found  in  ruptured  material 
an  exploration  would  most  likely  show  similar  areas  throughout 
the  region  of  fracture. 


10  MICROSCOPIC  EXAMINATION  OF  STEEL 

The  inclusion  of  oxide  is  frequently  accompanied  by  partial 
decarbonization,  -  and  is  most  frequently  found  on  or  near  the 
surface.  An  example  of  this  is  shown  in  photograph  No.  24, 
in  which  it  is  seen  that  a  crack  has  penetrated  through  the  slag, 
probably  mill  scale,  and  in  the  crack  are  the  remnants  of  the  slag. 
On  the  left  of  the  crack  is  the  normal  structure  and  on  the  right 
is  a  decarbonized  area  containing  slag.  Such  an  area  would 
probably  be  large  enough  to  show  to  the  naked  eye  and  would 
be  considered  unsafe.  Other  examples  of  oxide  inclosures  are 
cited  on  pages  14  and  15,  in  the  discussion  of  the  failure  of  10- 
inch  gun  and  1 4-inch  gun  lever. 


STREAKS. 

On  machining  a  metal  there  may  often  be  seen  lines  or  areas 
on  the  surface  which  have  a  different  luster  from  the  main  mass 
of  metal,  and  which  frequently  machine  quite  differently  from 
the  metal  surrounding  it;  or  on  polishing  and  etching,  lines  of 
different  luster  may  stand  out  prominently  and  invariably  elon- 
gated in  the  direction  of  forging.  Such  lines  or  areas  are  known 
as  streaks  or  ghost  lines,  and  may  occur  as  either  dark  or  light 
in  comparison  to  the  surrounding  metal.  Streaks  are  usually 
accompanied  by  slag.  If  the  slag  is  oxidizing  in  character,  the 
streak  will  be  decarbonized,  and  at  the  same  time  the  ferrite 
areas  will  be  embrittled  by  being  rich  in  phosphorus.  Such 
streaks  may  be  the  source  of  fracture,  and  should  be  looked  upon 
with  suspicion. 

Dark-colored  streaks  usually  contain  silicate  of  manganese 
and  are  less  harmful,  the  metal  showing  considerable  ductility 
in  the  streaked  region. 

Radial  test  pieces  from  gun  forgings  will  frequently  show 
streaks,  and  as  the  tension  is  normal  to  the  direction  of  exten- 
sion the  specimen  will  show  low  ductility.  Photographs  Nos.  25 
and  26  show,  respectively,  dark  and  light  streaks,  and  photo- 
graph No.  27  shows  the  microstructure  of  a  portion  of  the 
light-colored  streak  with  its  decarbonization  and  accompanying 
slag.  (See  also  Tests  of  Metals  for  1909,  vol.  n.) 


MICROSCOPIC   EXAMINATION  OF  STEEL  II 


HEAT  TREATMENT. 

It  is  important  that  the  heat  treatment  should  represent  the 
best  practice.  This  will  vary  with  the  purposes  for  which  the 
material  is  to  be  used.  In  annealed  steels  the  structure  should 
not  only  show  the  finest  grain  which  it  is  possible  to  give  the 
material  but  it  should  be  uniform  throughout  the  metal,  con- 
sidering, however,  that  the  mass  of  metal  influences  the  structure. 
Thus  the  surface  indications  are  not  always  the  same  as  would 
be  found  in  the  center  of  a  large  section.  Variations  between 
the  exterior  and  interior  should  therefore  be  expected  on  account 
of  slower  cooling  of  the  interior  and  the  inability  to  always  have 
the  forging  affect  the  whole  mass  of  metal  alike.  Coarse  struc- 
ture on  the  surface  will  usually  indicate  coarser  material  in  the 
center  of  the  mass.  Coarse-structured  material  may  pass  physi- 
cal specifications,  but  nevertheless  it  does  not  represent  the  best 
practice.  Photograph  Nos.  28  and  29  represent,  respectively, 
coarse  structure  and  fine  structure  of  the  same  material,  and  the 
latter  is  undoubtedly  the  better  material,  as  may  be  seen  from 
the  respective  physical  properties.  Photograph  No.  30  shows 
an  undesirably  coarse  structure,  and  photograph  No.  31  shows 
a  crystalline  structure  which  is  not  only  undesirable  but  which 
could  have  been  avoided  by  suitable  heat  treatment. 

Failure  to  obtain  uniformity  of  structure  is  shown  in  photo- 
graph No.  32.  Discussion  of  photographs  Nos.  49,  51,  and  53 
will  be  found  under  the  discussion  of  some  of  the  special  cases, 
viz.,  the  failure  of  i4-inchgun  lever  arm.  Dangerous  overheat- 
ing is  shown  in  photograph  No.  33.  In  this  case  the  material 
is  burnt,  as  is  shown  by  the  oxide  between  the  granules  of  pearlite. 

In  material  which  has  been  quenched  and  subsequently  tem- 
pered there  is  greater  difficulty  in  defining  the  structure,  but  in 
general  it  should  be  fine  and  usually  sorbitic,  unless  in  unusual 
cases  especially  high  strength  and  lower  ductility  is  desired,  in 
which  cases  troostite  may  be  found.  In  steels  in  which  it  is 
desired  to  have  high  strength  and  high  ductility  and  obtained 
by  either  oil  or  water  quenching,  followed  by  a  tempering  proc- 
ess, and  in  which  the  structure  is  usually  sorbitic,  it  should  be 


12  MICROSCOPIC   EXAMINATION  OF   STEEL 

observed  that  the  original  martensitic  structure  is  completely 
removed  by  the  tempering  process.  This  point  is  illustrated  by 
photograph  No.  34  in  which  there  are  distinct  remains  of  the 
original  martensitic  structure.  Such  a  steel  properly  heat  treated 
ought  to  show  a  structure  similar  to  photographs  Nos.  12  or  13. 

COMPOSITION. 

On  slowly  cooled  or  annealed  steels  the  percentage  of  carbon 
may  be  roughly  estimated.  On  quenched  steels  this  estimation 
is  much  more  difficult  and  usually  impossible.  However,  with 
some  experience  the  observer  can  tell  approximately  whether 
or  not  the  percentage  of  carbon  is  what  the  specifications  call 
for.  The  chemical  analysis,  however,  gives  more  accurate  data 
and  should  always  be  relied  upon  for  final  decision.  In  certain 
cases,  however,  a  rough  microscopic  analysis  may  be  made  with 
advantage.  For  example,  in  photograph  No.  35  (broken  recoil 
spring)  there  is  plainly  an  excess  of  cementite,  indicating,  as 
was  subsequently  shown  by  analysis,  about  1.25  per  cent  of  car- 
bon. Such  material  is  evidently  unsuited  for  spring  steel  on 
account  of  the  presence  of  the  weak  and  very  brittle  constituent, 
cementite.  The  main  body  of  this  spring  showed  troostite, 
not  in  itself  unsuitable,  most  of  the  carbon  being  in  solid  solu- 
tion, but  the  center  portion  was  not  cooled  rapidly  enough  to 
prevent  the  separation  of  free  cementite.  The  conclusion  is 
evident  that  a  steel  containing  less  carbon  would  have  been 
more  suitable. 

THE  EFFECT  OF  WORK  ON   GRAIN   SIZE. 

Steel  cooled  slowly  and  undisturbed  from  a  high  temperature 
will  show  a  coarsely  granular  or  crystalline  structure,  and  the 
size  of  the  grain  is  a  function  of  the  temperature  and  the  time 
during  which  the  material  was  held  at  the  maximum  tempera- 
ture, and  the  rate  at  which  the  material  was  cooled.  In  large 
masses  of  material  the  structure  will  be  coarser  in  the  center 
than  at  the  surface,  due  to  the  difference  in  rate  of  cooling.  In 
order  to  overcome  this  difference  and  at  the  same  time  to  produce 


MICROSCOPIC  EXAMINATION  OF  STEEL  13 

a  homogeneous,  uniform  material,  the  steel  is  worked  during 
the  period  at  which  grain  growth  would  ordinarily  take  place. 
Steel  which  has  been  hot- worked  down  to  the  Ari  point  will  show 
a  finer  grain,  and  will  be  stronger  than  the  same  steel  slowly  cooled 
without  work,  and  will  at  the  same  time  show  high  ductility. 
Examples  of  steel  worked  and  unworked  are  shown  in  photo- 
graphs Nos.  28  and  29.  Steel  which  has  been  worked  below 
Ari  —  cold-worked  —  will  show  considerable  distortion  of  grain, 
as  shown  in  photograph  No.  36. 

The  following  illustrations  will  serve  to  indicate  types  of  struc- 
ture which  have  been  found  in  defective  material  which  failed 
in  service. 

10-INCH  RIFLE,   MODEL  OF   1895. 

The  tube  of  this  gun  was  found  to  be  cracked  after  proof-firing. 
The  crack  was  situated  about  2  feet  in  from  the  breech  end  and 
extended  in  the  form  of  a  crescent  through  a  visible  distance 
of  about  10  inches.  At  the  central  portion  of  the  crack  the  walls 
were  considerably  more  separated  than  at  either  end,  and  con- 
siderable metal  had  been  removed  at  this  point.  The  general 
appearance  is  shown  in  photograph  No.  37.  It  will  be  noted 
that  the  crack  is  not  continuous. 

Photographs  Nos.  38  and  39  show  the  macrostructure  of  this 
steel  when  etched  with  iodine,  the  specimens  shown  in  photo- 
graph No.  38  being  taken  longitudinally,  radially,  and  tangen- 
tially.  The  segregation  of  phosphorus  is  indicated  by  the  dark 
lines  in  pieces  Nos.  2  and  5  of  photograph  No.  38  and  in  both 
pieces  of  photograph  No.  39  and  by  the  dark  spots  in  piece  No. 
3.  Photograph  No.  39  shows  the  metal  cut  from  the  region  of 
the  fracture  and  treated  in  the  same  manner,  as  shown  in  test 
piece  marked  5  in  photograph  No.  38,  and  in  other  test  pieces 
not  shown.  The  dark  streaks  were  found  to  be  elongated  paral- 
lel to  the  direction  of  forging,  and  it  is  reasonable  to  suppose 
that  the  elongation  will  always  be  parallel  to  the  surfaces  being 
forged.  By  inspection  of  the  two  larger  pieces  of  metal  cut  from 
the  region  of  the  crack  (photograph  No.  39)  it  will  be  seen  that 
there  is  considerable  distortion  of  the  phosphide  areas  near  the 


14  MICROSCOPIC   EXAMINATION  OF  STEEL 

rifled  surface.  This  would  seem  to  indicate  that  by  some  means 
a  fold  in  the  metal  had  taken  place  at  this  point  and  that  in 
forging  it  out  the  phosphide  areas  had  been  distorted  to  the  same 
extent  as  the  surface  of  the  fold. 

In  photographs  of  test  piece  No.  3  is  shown  a  face  cut  normal 
to  the  direction  of  forging.  In  all  specimens  cut  in  a  similar 
direction  there  is  always  the  same  evidence  of  ingot  structure  or 
primary  crystallization,  which  is  indicated  by  the  appearance  of 
the  white  interpenetrating  lines  as  shown.  This  crystallization 
was  produced  at  the  time  of  the  passage  of  the  metal  from  the 
liquid  to  the  solid  state  and  during  the  cooling  from  the  tempera- 
ture of  solidification  to  about  700°  C.  This  is  characteristic 
of  ingot  crystallization. 

Subsequent  heat  treatment  and  forging  should  have  com- 
pletely eliminated  all  traces  of  this,  and  its  presence  is  an  in- 
dication of  faulty  manipulation.  Further  evidence  of  faulty 
heat  treatment  is  found  in  the  microstructure  of  the  steel. 

Examination  of  sections  in  the  vicinity  of  the  crack  showed 
many  fine  cracks,  slag  inclusions,  and  carbonless  areas.  The 
cracks  led  to  slag,  and  where  the  slag  was  found  there  were  also 
found  decarbonized  areas.  Photograph  No.  40  shows  a  crack 
ending  in  slag  and  surrounded  by  decarbonized  iron.  The  course 
of  the  slag  was  quite  irregular,  as  shown  in  photographs  Nos.  41 
and  42. 

That  the  slag  originated  and  was  folded  into  the  metal  dur- 
ing the  process  of  forging  rather  than  during  the  manufacture 
of  the  steel  is  indicated  by  the  fact  that  it  is  strongly  oxidizing 
in  character.  If  it  had  originated  at  the  time  of  making  the 
steel,  and  had  then  burned  out  some  of  the  carbon,  there  would 
still  have  been  plenty  of  time  for  more  carbon  to  have  diffused 
back  into  these  areas.  It  is  found  here,  however,  surrounded 
by  free  ferrite  along  the  slag  lines,  as  shown  in  photograph  No. 
43  and  in  photograph  No.  44,  in  which  is  shown  an  isolated  slag 
spot  surrounded  by  free  ferrite,  and  this  in  turn  by  the  normal 
structure. 

Examination  was  made  of  the  surfaces  normal  to  the  lands  and 
grooves  of  the  rifling,  and  there  was  no  evidence  of  hardening 


MICROSCOPIC   EXAMINATION  OF   STEEL  15 

of  the  surface.     It  is  thus  highly  improbable  that  a  crack  could 
have  originated  from  the  presence  of  hardened  metal. 

It  is  believed  that  the  crack  in  this  gun  was  caused  by  folding 
in  of  some  of  the  metal  during  forging.  Incidentally  some  other 
defects  are  pointed  out. 

14-INCH  GUN  LEVER. 

This  lever  was  made  of  cast  steel  and  was  broken  during  the 
test  of  the  carriage.  A  microscopic  examination  revealed  the 
structures  shown  in  photographs  Nos.  46  to  51,  inclusive. 

Photograph  No.  46  shows  a  large  amount  of  segregated  ferrite 
with  included  slag.  These  ferrite  areas  are  of  such  a  size  that 
they  are  easily  visible  to  the  eye. 

In  the  fractured  ends  of  the  test  pieces  the  metal  shows  de- 
cidedly bright  spots,  and  these  are  due,  it  is  believed,  to  the 
presence  of  the  segregated  ferrite.  Further,  it  has  been  noticed 
that  along  the  stem  of  the  test  pieces  incipient  cracks  are 
developed,  as  shown  in  photograph  No.  45.  It  is  highly  probable 
that  these  incipient  fractures  are  developed  in  such  areas  as  shown 
in  photograph  No.  47,  which  shows  a  large  ferrite  field  at  the 
edge  of  the  specimen.  It  is  possible  that  if  the  forces  are  trans- 
mitted equally  throughout  the  test  piece  that  such  a  portion 
would  yield  more  readily  where  the  carbon  was  lowest.  The 
strength  of  such  a  spot  would  be  approximately  that  of  pure  iron, 
or  about  40,000  pounds  per  square  inch,  while  the  darker  areas 
which  contain  more  carbon  would  show  considerably  higher 
values.  The  presence  of  much  slag  in  the  ferrite  areas  would 
also  increase  the  brittleness  at  this  point,  and  would  account 
for  the  low  ductility  in  some  specimens.  Further  examples  of 
this  form  of  segregation  are  shown  in  photographs  Nos.  48  and 
49. 

The  metal  has  not  been  annealed  in  such  a  way  as  to  remove 
the  original  crystallization  which  took  place  at  the  time  of 
solidification.  The  crystalline  structure  is  shown  in  photo- 
graphs Nos.  50  and  5 1 .  Proper  annealing  should  have  completely 
wiped  out  all  appearance  of  this  kind.  Evidence  of  this  crystal- 
line nature  was  found  in  all  specimens  examined,  and  it  is  highly 


1 6  MICROSCOPIC  EXAMINATION  OF   STEEL 

probable  that  wherever  such  structure  appears  casting  strains 
exist.  Proper  annealing  would  also  have  removed  all  initial 
strains  and  would  have  refined  the  structure. 

The  strength  and  ductility  have  undoubtedly  been  affected  by 
the  aforementioned  factors,  viz.,  segregated  ferrite,  crystalline 
structure,  and  abundance  of  slag. 

12-INCH  NAVY  GUN. 

A  section  of  the  metal  i  inch  square  was  cut  from  the  region 
of  the  fracture,  and  the  macrostructure  was  developed  by  etching 
with  iodine  solution.  Photograph  No.  52  shows  the  macrostruc- 
ture in  which  is  shown  several  streaks  running  parallel  to  the  di- 
rection of  forging  on  the  left-hand  face,  and  on  the  right-hand 
face  an  irregular  area  of  segregation  normal  to  this  surface. 
Small  fissures  were  visible  and  both  faces  indicated  the  presence 
of  slag.  Photograph  No.  53  shows  the  microstructure  on  the 
longitudinal  face  in  which  is  seen  slag  and  small  fissures.  Pho- 
tographs Nos.  54  and  55  show  an  unetched  and  etched  area  in 
the  longitudinal  face,  some  of  the  same  slag  areas  being  shown 
in  each,  and  the  lack  of  homogeneity  of  the  metal  is  evident. 

The  cause  of  rupture  is  believed  to  have  been  due  to  the  pres- 
ence of  a  zone  of  streaked  metal,  and  further,  that  this  resulted 
from  the  presence  of  slag  in  the  original  ingot. 

POLISHING. 

To  prepare  a  specimen  for  microscopic  examination  it  is  neces- 
sary to  polish  the  surface  until  a  perfectly  smooth  finish  free 
from  scratches  is  obtained.  Frequent  examination  under  the 
microscope  will  enable  the  operator  to  determine  when  the  pol- 
ishing should  end.  Small  specimens  are  easiest  to  handle,  and 
the  threaded  ends  of  broken  test  specimens  are  commonly  used. 

The  broken  ends  are  cut  off  with  a  saw  and  smoothed  with  a 
dead  smooth  file,  then  rubbed  on  a  piece  of  No.  120  emery  cloth 
and  later  on  a  piece  of  No.  oo,  care  being  taken  to  change  direc- 
tion frequently  so  that  the  scratches  are  at  right  angles  to  each 
other.  The  piece  is  then  rubbed  on  a  block  of  wood  covered 


MICROSCOPIC   EXAMINATION  OF   STEEL  17 

with  felt  or  canvas  and  moistened  with  a  preparation  of  flour 
of  emery  and  water.  This  is  usually  applied  with  a  fine  2 -inch 
brush. 

The  specimen  is  then  rubbed  successively  on  similar  blocks 
moistened  with  tripoli  and  jeweler's  rouge.  The  piece  should 
be  carefully  cleaned  before  changing  from  one  abrasive  to  an- 
other. 

If  power  is  available  it  will  be  found  much  easier  to  polish  the 
specimens  on  rapidly  revolving  wheels  covered  with  fine  duck  or 
broadcloth,  to  which  the  abrasive  is  applied  with  a  brush  or  in 
the  form  of  a  spray. 

After  polishing,  the  specimen  is  washed  carefully  in  running 
water  and  immediately  wiped  dry  with  a  towel  or  piece  of 
absorbent  cotton. 

ETCHING. 

An  alcoholic  solution  of  nitric  acid  is  the  most  common  re- 
agent used  for  etching,  and  is  prepared  by  adding  4  cubic  centi- 
meters of  nitric  acid  Sp.  Gr.  1.42  to  96  cubic  centimeters  of 
grain  alcohol. 

A  small  quantity  of  the  acid  solution  is  placed  in  a  porcelain 
dish,  and  the  polished  surface  of  the  prepared  specimen  im- 
mersed for  about  10  seconds,  the  specimen  being  agitated  to 
prevent  adhesion  of  gas  to  the  surface.  It  is  then  removed  and 
washed  thoroughly  in  running  water  or  in  alcohol  and  quickly 
dried,  either  by  a  soft  cloth  or  by  a  blast  of  air.  Examination 
under  the  microscope  will  determine  whether  the  etching  has 
been  carried  far  enough.  The  time  of  etching  will  vary,  depend- 
ing on  the  hardness  of  the  steel,  and  therefore  the  period  must 
be  learned  by  experience. 

A  5  per  cent  alcoholic  solution  of  picric  acid  is  sometimes  used 
for  a  reagent,  and  on  low-carbon  soft  steels  it  gives  practically 
the  same  indications  as  the  alcoholic  nitric  acid.  Its  use  on 
hardened  or  tempered  steels  is  not  recommended. 

Sodium  picrate  is  used  to  distinguish  between  ferrite  and  ce- 
mentite.  This  solution  is  made  by  dissolving  2  grams  of  picric 
acid  in  98  cubic  centimeters  of  a  solution  of  250  grams  of  caustic 


1 8  MICROSCOPIC  EXAMINATION  OF   STEEL 

soda  (NaOH)  dissolved  in  750  cubic  centimeters  of  water.  The 
polished  specimen  is  immersed  in  the  boiling  solution  for  5  to 
10  minutes,  when  the  cementite,  if  present,  assumes  a  blackish 
coloration.  Ferrite  remains  bright  when  boiled  in  sodium  pic- 
rate  solution. 

If  a  permanent  record  is  desired,  photomicrographs  are  made, 
the  magnification  being  always  marked  on  the  negative  and 
print.  It  will  be  found  that  polished  surfaces  will  oxidize  if 
exposed  to  the  air  or  to  moisture.  Specimens  should  be  washed 
in  alcohol  and  stored  in  air-tight  boxes. 

For  the  examination  of  large  pieces  of  metal,  such  as  guns, 
etc.,  it  will  be  sufficient  to  polish  spots  about  the  size  of  a  50- 
cent  piece.  This  can  be  done  with  any  of  the  portable  outfits 
now  on  the  market,  or  may  be  done  by  hand. 

"  The  study  of  the  following  books  is  recommended,  to  sup- 
plement the  information  contained  herein: 

Howe,  Metallography  of  Cast  Iron  and  Steel. 

Rosenhain,  Physical  Metallurgy. 

Gulliver,  Metallic  Alloys. 

Edwards,  Physico-Chemical  Properties  of  Steel. 

Desch,  Metallography. 

Osmond  and  Stead,  Microscopic  Analysis  of  Metals. 

In  conclusion  it  may  be  stated  that  there  is  a  tendency  to 
over-rate  the  application  of  metallography.  Used  in  connec- 
tion with  the  physical  tests  and  chemical  analyses  it  furnishes 
valuable  supplementary  data.  The  inspector  is  warned  to  be 
conservative  in  his  conclusions,  at  least  until  he  has  acquired 
sufficient  experience  in  the  examination  of  steels  to  make  his 
conclusions  agree  with  the  facts. 

It  will  be  found  that  by  the  examination  of  specimens  whose 
heat  treatment  is  definitely  known,  the  inspector  will  soon  ac- 
quire a  fund  of  valuable  information  which  will  enable  him  to 
diagnose  an  unknown  steel  correctly.  Defective  metal  should 
be  examined  and  its  characteristics  noted  whenever  and  where- 
ever  it  is  found. 


PHOTOGRAPH  NO.  I.    X75. 
0.06%  carbon.     Approximately  pure  ferrite. 


- 

•? 


PHOTOGRAPH   NO.  2.     X75. 
0.18%  carbon.     Ferrite  (white)  and  pearlite  (dark). 


PHOTOGRAPH  NO.  3.     X76. 
0.32%  carbon.     Ferrite  (white)  and  pearlite  (dark). 


PHOTOGRAPH  NO.  4.     X75. 
0.49%  carbon.     Ferrite  (white)  and  pearlite  (dark). 


PHOTOGRAPH   NO.  5.     X75. 
0.57%  carbon.     Ferrite  and  pearlite. 


PHOTOGRAPH   NO.  6.     X75. 
0,71%  carbon.     Ferrite  and  pearlite. 


PHOTOGRAPH   NO.  7.     X75. 
0.83%  carbon.     Pearlite. 


PHOTOGRAPH   NO.  8.     X485. 
0.83%  carbon.     Pearlite. 


PHOTOGRAPH   NO.  9.     X75. 
1.22%  carbon.     Pearlite  and  cementite  (white). 


PHOTOGRAPH   NO.  10.     X75. 
1.46%  carbon.     Pearlite  and  cementite  (white). 


PHOTOGRAPH  NO.  II.    X75. 
Martensite. 


PHOTOGRAPH   NO.  12.     X75. 
Troostite. 


PHOTOGRAPH   NO.  13.     X75. 
Sorbite. 


PHOTOGRAPH  NO.  14.  X5O. 

Ferrite  and  massive  silicate  of  manganese.     Etch  figures  in  the 
silicate.     (Ferrite,  white;  silicate,  dark.) 


PHOTOGRAPH  NO.  15.  X50. 

Etch  figures  in  silicate  of  manganese,  and  cavities  due  to 
removal  of  silicate. 


PHOTOGRAPH   NO.  16.     X50. 
Sulphide  of  manganese  in  gun  steel. 


PHOTOGRAPH   NO.  17.     X50. 
Slag  elongated  in  direction  of  forging.     Specimen  unetched. 


PHOTOGRAPH   NO.  18.     X50. 
Unevenly  distributed  slag.    Specimen  unetched. 


PHOTOGRAPH   NO.  19.     ONE-THIRD  SIZE. 
Ingot  structure  in  steel. 


PHOTOGRAPH   NO.  20. 
Cross  section  of  steel  rail  showing  segregation  in  center  of  head,  web,  and  base. 


PHOTOGRAPH  NO.  21. 

Segregation  of  phosphorus  in  cold-rolled  shafting. 
Etched  with  iodine.     (White  areas.) 


PHOTOGRAPH  NO.  22. 

Cross  section  of  No.  21.     Etched  with  nitric  acid. 
(Dark  streaks.) 


PHOTOGRAPH   NO.  23. 
Area  of  strained  metal  brought  out  by  etching  with  copper  ammonium  chloride. 


PHOTOGRAPH   NO.  24.     X50. 
Crack,  slag,  and  decarbonized  area  found  on  the  base  of  a  steel  rail. 


PHOTOGRAPH   NO.  25. 
Dark  streaks  in  steel  rail. 


PHOTOGRAPH   NO.  26. 
Light  streaks  in  steel  rail. 


PHOTOGRAPH   NO.  27.     X50. 
Microstructure  of  light-colored  streak  In  No.  26. 


PHOTOGRAPH   NO.  28.     X75. 
Microstructure  of  cast  steel  ingot  as  cast. 
Elastic  Limit  39.000 

Tensile  Strength       77.000 
Elongation  10.5 

Contraction  of  Area       16.9 


* 

v 


PHOTOGRAPH   NO.  29.     X75. 

Microstructure  of  cast  steel  ingot  forged  to  1450°  F.     (788°  C.) 
Elastic  Limit  50,500 

Tensile  Strength        83.600 
Elongation  27.5 

Contraction  of  Area      43.3 


PHOTOGRAPH   NO.  3O.     X75. 
Coarse  microstructure  in  gun  steel. 


PHOTOGRAPH   NO.  31.     X5O. 
Crystalline  structure  in  cast  steel. 


PHOTOGRAPH   NO.  32.     X75. 
Microstructure  showing  incomplete  heat  treatment. 


PHOTOGRAPH  NO.  33.     X75. 
Microstructure  of  overheated  steel. 


PHOTOGRAPH   NO.  34.     X500. 

Microstructure  of  forging  which  had  been  originally  martensitic.  and 
in  tempering  this  structure  has  not  been  completely  eliminated. 


PHOTOGRAPH  NO.  35.  X65. 

Microstructure  in  center  of  broken  recoil  spring  showing  excess 
cementite,  indicating  high  carbon.  Left,  white  lines  are  cementite 
etched  with  nitric  acid  solution;  right,  dark  lines  are  cementite 
etched  with  sodium  plcrate  solution. 


PHOTOGRAPH   NO.  36.     X50. 

Microstructure  of  steel  subjected  to  cold  work,  and 
showing  distortion  of  grain. 


s  . 

tl 

I    .5 

I  * 

5s 

fe  2 


u 
O     g 

Z      u 


§  ? 

O    .E 

3 

5 

3 
L 

*M 
O 

u 

O 

i 
I 


PHOTOGRAPH  NO.  40.     X50. 
Crack  and  slag,  with  decarbonized  area  in  10-inch  rifle. 


PHOTOGRAPH  NO.  41.     X50. 
Slag  in  10-inch  rifle,  unetched. 


PHOTOGRAPH   NO.  42.     X50. 
Slag  in  10-inch  rifle,  unetched. 


PHOTOGRAPH    NO.  43.     X50. 
Slag  and  decarbonized  area  in  10-inch  rifle. 


PHOTOGRAPH   NO.  44.     X50. 
Island  of  slag  in  decarbonized  area  in  10-inch  rifle. 


CIS.  Al. 

PHOTOGRAPH  NO.  45. 
Fourteen-inch  gun  lever  test  bar  specimens  showing  incipient  cracks. 


PHOTOGRAPH   NO.  46.     X50. 
Fourteen-inch  gun  lever.    Segregated  ferrite  (white)  with  slag  inclosures. 


PHOTOGRAPH   NO.  47.     X50. 

Fourteen-inch  gun  lever.     Segregated  ferrite  on  edge  of  test  piece.     Slag 
inclusions. 


PHOTOGRAPH   NO.  48.     X50. 
Fourteen-inch  gun  lever.     Segregated  ferrite  (white)  with  slag  inclusions. 


PHOTOGRAPH  NO.  49.     X5O. 
Fourteen-inch  gun  lever.    Segregated  ferrite  with  slag  inclusions. 


PHOTOGRAPH   NO.  50.     X50. 
Fourteen-inch  gun  lever.     Segregated  ferrite  and  crystalline  structure. 


PHOTOGRAPH   NO.  61.     X50. 
Fourteen-inch  gun  lever.    Crystalline  structure. 


PHOTOGRAPH   NO.  52. 
Macrostructure  of  metal  removed  from  12-inch  gun. 


PHOTOGRAPH   NO.  63.     X60. 
Twelve-inch  gun.     Microstructure  showing  slag  and  fissures. 


PHOTOGRAPH   NO.  54.     X50. 
Twelve-inch  gun.     Slag  in  unetched  specimen. 


PHOTOGRAPH   NO.  55.     X50. 
Twelve-inch  gun.    Same  area  as  No.  54,  etched  with  alcoholic  nitric  acid. 


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MAR  1  5  1951 


DEC  1 7  1954 
OCT281955 

JA'1       6      * 


LD  21-100m-9,'48(B399sl6)476 


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